A standard rotary kiln has a large-diameter cylindrical housing that is centered on an axis that is slightly inclined to the horizontal and that is rotated about this axis. Particles to be heat treated are fed into the upper upstream end of the housing and exit from the lower downstream end while being tumbled in the housing in contact with heat-exchange elements therein. Such a kiln can be used simply to dry sand or coal or can be used to calcine minerals, for instance in the production of cement.
German patents 19,079, 1,629,044, and 3,729,032 of F. Klee, J. Kaiser, and H. Luhmann, respectively, describe such a kiln provided internally with a succession of axially spaced radiators that are connected in parallel to each other to a hot-fluid feed line and a cool-fluid output line. Since the temperatures of the radiators are all substantially the same, the result is that the heat exchange is not very efficient in that the cold particles first hit the hot upstream radiator and as they are heated move downstream past radiators at the same temperature until they exit the machine. The heat exchange is greatest at the upstream radiator and least at the downstream one so that overall exchange efficiency is unsatisfactory in that the liquid leaving the downstream heat exchanger has not given up much of its heat.
German patent document 3,644,806 of H. Klutz describes a method for drying brown coal wherein condensed vapors of a fluidized bed dryer are fed through radiators of the fluidized bed and are condensed therein so that at least a part of the condensate can be used for preheating the brown coal to be dried. The condensate has a temperature of about 111.degree. C. and should be cooled to about 40.degree. C. The heat recovered in this manner is used to prewarm the moist brown coal from 15.degree. C. to 65.degree. C.
In this system to transmit the heat of the condensate to the brown coal it is necessary to use an apparatus of very large size with big heat-exchange surfaces. This is needed on the one hand because of the relatively small temperature differential and on the other hand because of the considerable throughput which in this case is 55t/h and which can be even more.
USSR patent document 918,738 of I. Pievskii describes another kiln having a succession of radiators connected in series. The heat-exchange medium moves through the radiators co-directionally with the particles through the exchanger, that is both the heat-exchange liquid and the particles move from the upstream end to the downstream end of the kiln. Thus as the particles get hotter the heat-exchange fluid gets cooler. As a result heat-exchange efficiency drops off because the temperature differential decreases.